(1) Field of the Invention
This invention relates to a III nitride film and a III nitride multilayer structure, particularly usable for a semiconductor light-emitting element such as a light-emitting diode or a semiconductor element such as a high velocity IC chip.
(2) Related Art Statement
III nitride films are used as semiconductor films for light-emitting diodes, and recently get attention as semiconductor films for high velocity IC chips for cellular phones. In view of application for field emitters, much attention is particularly paid to Al-including III nitride films.
Conventionally, such an Al-including film is fabricated by a MOCVD method in which trimethyl aluminum (TMA) or triethyl aluminum (TEA) is employed as an Al raw material gas, and ammonia (NH3) is employed as a nitrogen raw material gas. In this case, a substrate on which the III nitride film is formed is set on a susceptor installed in a reactor and is heated to around 1000xc2x0 C. by a heater provided in or out of the susceptor. Then, the Al raw material gas, the nitrogen raw material gas and another raw material gas including other additive elements are introduced with a given carrier gas into the reactor and supplied onto the substrate.
The raw material gases thermochemically react on the substrate, and the thus decomposed constituent elements chemically react to deposit a desired III nitride film on the substrate. In order to obtain a semiconductor element as designed, it is required that the crystallinity of the III nitride film should be improved.
FIG. 1 is a perspective view showing a crystal structure unit of a normal III nitride such as a GaN film, and FIG. 2 is a graph showing the relation in X-ray rocking curve between the (100) plane and the (002) plane of the III nitride films.
The X-ray rocking curve is normally measured by means of a method disclosed in xe2x80x9cJ. Appl. Phys. Vol. 38(1999), pp. L611-613xe2x80x9d. In the measuring method, an X-ray is introduced onto a crystal plane to measure the X-ray rocking curve such that the X-ray is reflected symmetrically from the crystal plane. In the above document, full widths at half maximum (FWHMs) X-ray rocking curve values for various crystal planes of the III nitride film are listed in Table. 1. It is reported in the document that the Twist component corresponding to the FWHM X-ray rocking curve value at the (100) plane is 0.33 degrees (=1188 seconds) and the Tilt component corresponding to the FWHM X-ray rocking curve value at the (002) plane is 0.09 degrees (=324 seconds), in the case that GaN is fabricated without the application of special growth techniques such as the ELO technique.
In view of the crystallinity of the III nitride film, the mosaicity at the (001) plane (or (002) plane) and the mosaic property at the (100) plane parallel to the C-axis of the III nitride film must be considered. The crystallinity of the GaN film grown without the application of special growth techniques such as the ELO technique satisfies the relation shown in FIG. 2.
The crystallinity of the Al-including III nitride film does not satisfy a specific relation as shown in FIG. 2, and it is considered that the crystallinity of the Al-including III nitride film is relatively inferior to that of the GaN film because it has difficulty in being formed in high crystallinity, as compared with the GaN film.
In the case of the low crystallinity of the III nitride film, the FWHM X-ray rocking curve value at the (002) plane and the FWHM X-ray rocking curve value at the (100) plane satisfies the relation of region B. On the other hand, in the case of the high crystallinity of the III nitride film, the FWHM in X-ray rocking curve at the (002) plane and the FWHM X-ray rocking curve at the (100) plane satisfies the relation of region A.
In the region B, therefore, the FWHM X-ray rocking curve value at the (100) plane increases as the FWHM X-ray rocking curve value at the (001) plane increases. That is, the crystallinity of the (100) plane increases as the crystallinity of the (001) plane increases.
In the region A, however, the tendency of the FWHM X-ray rocking curve value at the (100) plane is contrary to that of the FWHM X-ray rocking curve value at the (001) plane. That is, the FWHM X-ray rocking curve value at the (100) plane increases as the FWHM X-ray rocking curve value at the (001) plane decreases, so the crystallinity of the (100) plane increases as the crystallinity of the (001) plane decreases.
It is obviously required that the III nitride film has a high crystallinity so as to construct a semiconductor element, and as a result, in many cases, the relation between the crystallinities of the (100) plane and the (001) plane are dominated in the region A shown in FIG. 2. In this case, conventionally, the crystallinities of the (100) plane and the (001) plane can not be improved simultaneously, and thus, balanced in a given condition, to maintain the total crystallinity of the III nitride film in a given condition.
It is an object of the present invention to provide an Al-including III nitride film which has the higher crystallinities of the (100) plane and the (001) plane simultaneously, and a III nitride multilayer usable as an underlayer for a semiconductor element which utilizes the III nitride film.
In order to achieve the above object, this invention relates to a III nitride film including a full width at half maximum X-ray rocking curve value of 800 seconds or below at the (100) plane, and a full width at half maximum X-ray rocking curve value of 200 seconds or below at the (002) plane.
In a conventional III nitride film, if the FWHM X-ray rocking curve value at the (002) plane, that is, at the (001) plane is set to 200 seconds or below, the FWHM X-ray rocking curve value at the (100) plane is beyond 800 seconds, and for example, set to around 1000 seconds. In this case, therefore, the crystallinity of the (100) plane can not be enhanced sufficiently.
In the III nitride film of the present invention, on the contrary, even though the FWHM X-ray rocking curve value at the (002) plane is set to 200 seconds or below, the FWHM X-ray rocking curve value at the (100) plane can be set to 800 seconds or below. Therefore, the cystallinities of the (100) plane and the (001) plane can be developed simultaneously.
In a preferred embodiment of the III nitride film of the present invention, the FWHM X-ray rocking curve value at the (100) plane is set to 500 seconds or below, and the FWHM X-ray rocking curve value at the (002) plane is set to 150 seconds or below. That is, even though the FWHM X-ray rocking curve value of the (002) plane is set to 500 seconds or below so as to develop the crystallinity of the (001) plane, the FWHM X-ray rocking curve value at the (100) plane can be set to 500 seconds or below, to develop the crystallinity of the (100) plane sufficiently.
The III nitride film satisfying the requirement of the present invention may be fabricated as follows. First of all, a III nitride underfilm having an Al content of 50 atomic percent or more for all of the III elements is formed in a given thickness on a given substrate by means of a CVD method. Then, the III nitride film is formed on the III nitride underfilm so that the Al content of the III nitride film is set lower than the Al content of the III nitride underfilm by 10 atomic percent or more. In this case, the crystallinity of the III nitride film is improved, originated from the difference in Al content between the III nitride underfilm and the III nitride film, so that both of the crystallinities of the (100) plane and the (001) plane can be developed.
This invention also relates to a III nitride multilayer structure including a given substrate, a III nitride underfilm including an Al content of 50 atomic percent or more for all of the III elements, and a III nitride film including a lower Al content than the Al content of the III nitride underfilm by 10 atomic percent or more. A full width at half maximum X-ray rocking curve value of the III nitride film is set to 800 seconds or below at the (100) plane. A full width at half maximum X-ray rocking curve value of the III nitride film is set to 200 seconds or below at the (002) plane. The III nitride multilayer structure is made through the fabricating process of the III nitride film of the present invention.
The III nitride multilayer structure includes the high crystallinity III nitride film on the top, so it is preferably usable as an underfilm for a semiconductor element.